144 
margin, which cannot be seen when looking at the shell from the side of the fixed valves, 
but which is very distinct when the valve is isolated and looked at from the inner side (Pl. XI, 
fig. 12). The third part lies between the two, its shape is also triangular and it extends from 
the apex to the lateral margin of the valve. In this part the growth ridges run nearly 
longitudinally, but as the valve is here somewhat hollowed out longitudinally, the course of 
the growth ridges is not straight but undulating. 
The fixed tergum is composed of two parts standing at right angles to one another: 
the part at the hinder portion of the valve, which is overlapped by the tergal margin of the fixed 
scutum, is the larger one, and has an irregular quadrangular shape. Its upper margin forms part 
of the occludent margin and is relatively short; its basal margin is considerably longer and 
distinctly undulating. It has a very irregular scutal margin (Pl. XI, fig. 13) and its fourth margin 
forms the edge of the angle with the other part of the valve. The shape of the latter is 
triangular, it has a short basal margin and its free lateral margin lies partly as occludent margin 
along the larger occludent margin of the moveable tergum, partly along the lateral margin of 
the carina. This part of the valve is like the other but is more strongly ribbed longitudinally, 
the ribs growing more prominent towards the base of the valve. 
Size. The distance from the apex of the rostrum to that of the carina is in one of 
the largest specimens 6 mm.; the greatest height of the same shell is 4.5 mm., the greatest 
dimension of the base, measured along the spine to which it is attached, is 8.3 mm. 
The study of the structure of the animal’s body has given the following results: 
Mouth much flattened laterally. Labrum rounded anteriorly but not bullate. Crest with a 
row of teeth, about 15 on each side; ten of these on each side from the middle have the apex with 
two or even three points, the remaining five at both extremities of the crest having the ordinary 
triangular shape, the most distal teeth being very small. Palpi rather large, broad at the 
base, bluntly pointed at the extremity, with a row of short bristles along the inner margin and 
a tuft of longer hairs at the extremity. A few short bristles scattered over the outer surface. 
Mandible (PI. XIII, fig. 11) much as usual in this genus; distance between tips of first 
and second teeth almost twice as long as that between second and third. Inferior angle moderately 
produced, terminating in a group of four small teeth. Teeth on the front side of the inferior 
angle at their bases fused together in a kind of crest, the tips of five or six of them emerging 
above the margin of the crest. 
Maxilla (Pl. XII, fig. 12) with short spines and a very shallow notch. Of the three 
upper spines the third is very delicate and lies against the second. 3 or 4 small spines are 
disposed in the notch, whereas the somewhat projecting inferior part bears half a dozen of spines, 
of which one is longer and slightly stouter; a group of numerous small spinelike hairs occupies 
the edge of the inferior angle. 
Second maxillae small, outer margin rounded, inner margin with a shallow notch, 
and indistinctly bilobed. Surface with short hairs as usual. 
Cirri. Cirrus of first pair with very unequal rami: the long ramus has 21 segments and 
is slender, the short one has 9—10 segments, and is broad and somewhat curved. Of the latter 
all the segments are densely clothed with hairs; of the long ramus the seven lower segments 
16 
